The present invention relates to a piston and cylinder assembly.
Piston and cylinder assemblies are commonly used either as fluid motors or as fluid pumps. When a piston cylinder assembly is being used as a pump, a piston is usually moved relative to a cylinder to expel fluid from the cylinder at a predetermined rate with each increment of movement of the piston relative to the cylinder. However, under certain circumstances, it is advantageous to have fluid expelled from the cylinder at a first rate during a first portion of the movement of a piston and to have fluid expelled from a cylinder at a second rate during a second portion of the movement of the piston.
A two stage piston and cylinder assembly is disclosed in U.S. Pat. No. 2,434,296. This patent discloses a manually actuated pump in which a control handle can be raised against the influence of a biasing spring then moved downwardly to move a relatively small inner piston relative to an outer piston and a cylinder. As this occurs, fluid is expelled from the cylinder at a first, relatively low rate. As the handle continues to be lowered, a second, relatively large piston, moves with the first piston. As the two pistons move downwardly together, fluid is expelled from the cylinder at a second rate which is greater than the first rate.
A press assembly having upper and lower draw rings which grip a workpiece is disclosed in U.S. patent application Ser. No. 429,200 filed Oct. 30, 1989 by John Terrell, Paul Kadis, Leonard Hiney, and Susan Pfaff and entitled "Press Assembly and Method of Operation." During opening of the press assembly, the lower draw ring is accelerated by a control assembly before the upper and lower draw rings move together. During closing of the press assembly, the control assembly effects deceleration of the lower draw ring.